DEVELOPMENT OF LAMPREY 



191 



future dorsal region of the embryo ; on this side the 

 margin of the blastopore is known as the dorsal lip, DLy 

 while to the right the ventral lip is seen greatly enlarged 

 by the yolk-bearing cells, Y. A somewhat later stage 

 (Fig. 208) shows the blastopore as a narrowly constricted 

 opening, BP, whose dorsal lip is slightly raised at its left- 

 hand margin. The head of the embryo is to arise near 

 the opposite pole (as in Fig. 210), and is thence to elon- 

 gate into neck and trunk (Fig. 212). A sagittal section of 

 a stage, slightly older than Fig. 208, shows admirably the 

 structures of the embryo that have thus far been differ- 

 entiated (Fig. 209). Contrasting with Fig. 207, it will 

 thus be seen that the coelenteron, arising at BP, has 

 become greatly elongated ; at its blind end its lining mem- 

 brane, entoderm, ENy is in contact with an indented por- 

 tion of the ectoderm, at 5, where later the opening of the 

 mouth will be established ; and that ventrally the coelen- 

 teron has given off a pouch which passes into the yolk, and 

 will later be differentiated as the liver. That the entire 

 dorsal wall of the coelenteron has become thickened, con- 

 stitutes the main difference between the sections of Figs. 

 207 and 209 ; there have, in other words, arisen between 

 the entoderm and ectoderm of Fig. 207 the central ner- 

 vous system, or medullary cord, M, and the notochord, CH. 

 The .origin of these structures may best be traced in the 

 cross-section of a slightly earlier stage (Fig. 213) ; the 

 coelenteron, or gut, is at G, the ectoderm at EC, the yolk 

 cells intervening at F; and the notochord and medullary 

 cord, CH, and M, in the sagittal region immediately be- 

 tween the gut and the ectoderm. In the medullary region 

 the ectoderm cells are seen pressed together, growing down- 

 ward and sidewise, forming altogether a compact cell cord * 



* As in Teleosts, but unlike other vertebrates. 



